Atrial fibrillation and arrhythmias are frequently caused by the presence of an arrhythmogenic substrate, such as ectopic foci, or an accessory atrioventricular pathway. Therapies have been developed for treating atrial fibrillation and arrhythmias by destroying cardiac tissue containing the arrhythmogenic substrate or accessory pathway responsible for the atrial fibrillation or arrhythmia. A variety of approaches have been taken, including application of electrical energy or other forms of energy to destroy the undesired cardiac tissue. As examples, ablation of cardiac tissue has been accomplished by application of radio frequency electrical current, microwave energy, heat, electrical pulses, ultrasound, cryothermy, and lasers to the tissue. At present, ablation using RF energy is perhaps the most widely practiced in the context of ablation procedures that can be carried out by a catheter inserted into the closed heart.
RF catheter ablation is generally performed after an initial mapping procedure where the locations of the arrhythmogenic substrates or accessory pathways are determined. After mapping, a catheter having a suitable electrode is introduced to the appropriate chamber and manipulated so that the electrode lies proximate the cardiac tissue to be ablated. RF energy is then applied through the electrode to the cardiac tissue in order to ablate a region of the tissue that forms part of the arrhythmogenic substrate or accessory pathway. By successfully destroying that tissue, the arrhythmogenic substrate or accessory pathway is destroyed so that the abnormal signaling patterns responsible for the atrial fibrillation or arrhythmia will no longer occur.
The RF energy delivered through the electrode causes tissue in contact with the electrode to heat through resistance of the tissue to the induced electrical current. Proper heating of the tissue causes ablation. Heating of the tissue and surrounding blood beyond a certain temperature, however, can cause desiccation or charring of the tissue, catheter adhesion to the charred tissue, and emboli development within the surrounding blood. All of these problems associated with overheating of the tissue and surrounding blood at the ablation site increase the risk of complication or death to the patient.
Additionally, blood and tissue overheating may cause a coagulum to form around the RF electrode. This coagulum greatly increases the impedance between the RF electrode and the return electrode, and proportionately reduces the RF energy delivered to the target tissue. RF energy that is delivered to the target tissue via the coagulum is insufficient to cause heating or ablation. Therefore, when a coagulum forms it must be detected so that the catheter can be removed from the patient and the coagulum cleaned off the electrode. If the coagulum is not detected, the procedure may fail to adequately destroy the arrhythmogenic substrate or accessory pathway that is the cause of the atrial fibrillation or arrhythmia.
One response to the problems caused by blood and tissue overheating has been the inclusion of a temperature sensor within the RF electrode, in conjunction with feedback control of the RF signal to maintain the blood and tissue temperature at a set level. The set level typically is below the temperature at which overheating is believed to occur. For example, U.S. Pat. No. 5,837,001 discloses a feedback network that controls the gain of an RF amplifier based on a comparison of the signal received from a temperature sensor to a preset temperature value.
Unfortunately, despite temperature controlled ablation, blood and tissue overheating still occurs during some RF catheter ablation procedures. Further, temperature controlled ablation does not detect coagulum formation. When a coagulum forms around an electrode, any subsequent RF applications result in heating of the coagulum, which in turn heats the RF electrode and the included thermocouple. During temperature controlled ablation, this leads to the maintenance of a high temperature with relatively low RF power output. High electrode temperature with relatively low RF power output is as indicative of good electrode-tissue contact and sufficient heating, as it is of coagulum formation, making the distinction difficult.
In realization that coagulum formation increases the impedance between the RF electrode and the return electrode, some existing techniques for detecting coagulum formation involve monitoring of the measured impedance. According to these techniques, if the impedance value exceeds a predetermined range, or increases at greater than a predetermined rate, an alarm may be communicated to the operator or RF power to the electrode may be reduced or suspended. In either case, the operator will be alerted to the need to remove the catheter and remove the coagulum.
Unfortunately, this method of coagulum detection may not be reliable. For example, if the coagulum is soft, the electrical properties are similar to the blood. The formation of a soft coagulum will not appreciably change the measured impedance, and will not be detected by the method of these references.
Further, coagulum detection may not reliably detect a coagulum early in the coagulum formation process. It is desirable to detect a coagulum early in the formation process. Occasionally, when the physician or technician removes the catheter to check for the presence of a coagulum or remove coagulum from the electrode, the coagulum breaks off of the electrode as the catheter is being removed from the patient. This broken-off coagulum may become an embolus, and poses a serious risk to the patient. The larger the coagulum is allowed to become, the greater the risk to the patient.
Commonly assigned U.S. Pat. No. 6,036,078, issued to Wittkampf, discloses a system and method for determining the degree to which the electrode is in contact with the cardiac tissue. In general, Wittkampf describes the repeated application of RF energy of low, non cell destructing power, and the monitoring of the temperature response at the electrode to the energy application. The temperature at the electrode in response to the energy application will be proportionately higher depending on the degree of electrode contact with the cardiac tissue.
Therefore, there is a need in the art for a system and method for reliably detecting the formation of a coagulum on an electrode during ablation. Further, there is a need in the art for a system and method for reliably detecting the formation of a coagulum on an electrode early in coagulum formation process.
Examples of RF ablation techniques and devices, including those employing temperature controlled ablation, impedance monitoring, and other methods for detecting or avoiding coagulum formation, may be found in the issued U.S. Patents listed in Table 1 below.
TABLE 1U.S. Pat. No.InventorIssue Date6,063,078WittkampfMay 16, 20005,971,980ShermanOct. 26, 19995,957,961Maguire, et al.Sep. 28, 19995,931,835MackeyAug. 3, 19995,843,075TaylorDec. 1, 19985,837,001MackeyNov. 17, 19985,779,699LipsonJul. 14, 19985,755,715Stern, et al.May 26, 1998
All patents listed in Table 1 above are hereby incorporated by reference herein in their respective entireties. As those of ordinary skill in the art will appreciate readily upon reading the Summary of the Invention, Detailed Description of the Preferred Embodiments and claims set forth below, many of the devices and methods disclosed in the patents of Table 1 may be modified advantageously by using the techniques of the present invention.